It has long been recognized that the energy expended in forming an re-forming articles of inorganic glasses is a significant cost factor. Moreover, the requirement of higher shaping temperatures demands the use of glass contact surfaces prepared from materials exhibiting greater refractoriness, which materials are typically more expensive. Furthermore, where a glass is to be utilized as a sealant, its transition temperature or transformation temperature (Tg) must be lower than the components being sealed together in order to avoid thermal deformation of the latter. Therefore, there has been considerable research to develop sealing glasses demonstrating transition temperatures below 450.degree. C., and preferably below 350.degree. C.
Because glass compositions containing silica as the principal network former or glass former typically exhibit transition temperatures substantially in excess of 450.degree. C., the search for glass compositions manifesting the desired low transition temperatures has customarily led to the use of B.sub.2 O.sub.3 and/or P.sub.2 O.sub.5 as the primary network forming component. U.S. Pat. No. 3,732,181 is illustrative of that practice. The patent discloses three broad areas of composition:
(1) PbO+P.sub.2 O.sub.5 .gtoreq.95 mole %, wherein PbO constitutes 20-80 mole %;
(2) PbO+R.sub.2 O (alkali metal oxides)+P.sub.2 O.sub.5 .gtoreq.95 mole %, wherein PbO comprises 5-60 mole %, R.sub.2 O constitutes 5-35 mole %, and P.sub.2 O.sub.5 is present up to 85 mole %; and
(3) PbO+R.sub.2 O+B.sub.2 O.sub.3 +P.sub.2 O.sub.5 .gtoreq.95 mole %, wherein PbO comprises 5-30 mole %, R.sub.2 O constitutes 5-30 mole %, B.sub.2 O.sub.3 composes 5-20 mole %, and P.sub.2 O.sub.5 comprises 15-85 mole %.
Although the above-cited composition ranges indicate the required presence of PbO, the patent asserts that part or all of the PbO may be replaced with divalent metal oxides.
Experience in the glass composition art has indicated that borate-based and phosphate-based glasses commonly demonstrate poorer chemical durability and resistance to attack by moisture than silica-based compositions, and that disadvantage becomes even more of a problem as the glasses are devised to manifest lower transition temperatures. For example, phosphate-based glasses demonstrating low transition temperatures are typically readily attacked by boiling water and, in not infrequent cases, exhibit hygroscopicity. As customarily defined, the transition temperature of a glass is the temperature at which increases in specific heat and thermal expansion coefficient take place which are accompanied by a sharp drop in viscosity.
Therefore, the principal objective of the present invention was to develop glasses exhibiting transition temperatures below 450.degree. C., most preferably below 350.degree. C., with working temperatures, i.e., temperatures at which the glass manifests a viscosity of about 10.sup.4 -10.sup.7 poises, below 500.degree. C., most preferably between about 350.degree.-450.degree. C., which glasses display remarkable chemical durability when contacted with boiling water.